Unfortunately there is no way to fix this since it is how repeating sections are designed.

To better understand why, you need to understand what the repeating section is doing. A repeating section is storing 1 or many instances of the field or fields you put within it.

When you're not using a repeating section, you're storing a single value into the field or list column.

For example, if you have a Choice field on your form with the options, "True" or "False", only one value is selected and stored in the field. So it is a 1 to 1 relationship between the Nintex form control and the list column / field.

When you put that Choice Field into a repeating section, you're changing it to a 1 to many relationship. Going back to the example, you can now have multiple sections each with a different value for the True / False choice field. You cannot easily store that into the associated field because the field is built to be 1 to 1, and you've just changed the controls behavior to be 1 to many. That is why it is disassociating your control from the field.

A repeating section stores its content (that includes the value of the fields within it) as XML in your list item. By default it goes into a hidden field called "Form Data", but you can associate a repeating section to store its XML into a multi-line of text field so you can view its content. Doing this allows you to then manipulate and use the values within the XML of the repeating section.

The following post has a bit more information if this post doesn't help.

Unfortunately there is no way to fix this since it is how repeating sections are designed.

To better understand why, you need to understand what the repeating section is doing. A repeating section is storing 1 or many instances of the field or fields you put within it.

When you're not using a repeating section, you're storing a single value into the field or list column.

For example, if you have a Choice field on your form with the options, "True" or "False", only one value is selected and stored in the field. So it is a 1 to 1 relationship between the Nintex form control and the list column / field.

When you put that Choice Field into a repeating section, you're changing it to a 1 to many relationship. Going back to the example, you can now have multiple sections each with a different value for the True / False choice field. You cannot easily store that into the associated field because the field is built to be 1 to 1, and you've just changed the controls behavior to be 1 to many. That is why it is disassociating your control from the field.

A repeating section stores its content (that includes the value of the fields within it) as XML in your list item. By default it goes into a hidden field called "Form Data", but you can associate a repeating section to store its XML into a multi-line of text field so you can view its content. Doing this allows you to then manipulate and use the values within the XML of the repeating section.

The following post has a bit more information if this post doesn't help.